DESCRIPTION (adapted from the applicant's description): The overall goal of this proposal is to elucidate the basis for structural organization of the calcium release units (CRUs) of cardiac and skeletal muscle which underlie excitation-contraction coupling. CRUs are comprised of junctional sarcoplasmic reticulum (jSR) which are composed of a luminal calcium-binding protein, calsequestrin (CSQ), and several membrane proteins which include the calcium-release channel (ryanodine receptor; RyRs), and supporting proteins junctin and triadin. The adjacent areas of sarcolemma include L-type calcium channels (dihydropyridine receptors; DHPRs). To determine the basis for the assembly, organization, and maintenance of structural integrity of these units, structure will be assessed by electron microscopy, using thin sections and freeze fracture methods, and protein composition will be assessed using immunofluorescence coupled to confocal microscopy and by Western blotting. Three strategies will be used to assess the effect of altered protein composition on the structure of CRUs. 1) Transgenic mice which overexpress one or more of the CRU proteins will be studied. 2) Muscle specific CRU proteins will be expressed in non-muscle cells in order to determine the proteins necessary for formation of CRUs. 3) The composition of CRUs in skeletal muscle of Amphioxus will be examined using molecular biology and ultrastructural approaches.